Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2003-01-15
2004-06-22
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S281000
Reexamination Certificate
active
06753997
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic field generator for devices utilizing magneto-optical effect, an optical device and optical attenuator which incorporate such a magnetic field generator, and a method of fabricating a base substrate for the magnetic field generator. More particularly, the present invention relates to a magnetic field generator which applies an arbitrary magnetic field distribution to a magneto-optical crystal, as well as to an optical device and optical attenuator incorporating such a magnetic field generator. It further relates to a method of fabricating a base substrate for that generator.
2. Description of the Related Art
Strenuous efforts have been made these days to develop high-bandwidth, high-speed data communications networks to meet the needs for realtime distribution of large amounts of data, including high-quality images and videos. Particularly, the use of the Internet is continuously expanding, and this situation raises an issue of how to handle the rapidly increasing network traffic. One approach is to increase the number of information-carrying channels that are multiplexed in a fiber optic cable. While there are several ways to achieve this, the wavelength-division multiplexing (WDM) is known as an especially promising technology for high-bandwidth data transport. WDM systems enable us to send multiple optical signals with different wavelengths over a single fiber, and they have actually been deployed in long-haul telecommunications network infrastructures.
In such long-haul optical networks, optical amplifiers should be placed midway to compensate for fiber loss. Some amplifies perform optical-to-electrical conversion to amplify the signal in electrical form, while others boost the intensity of optical signals all optically. The latter type is of greater interest these days because they can be implemented at lower costs than the former type.
The all-optical amplifiers, however, exhibit some non-linear response to different wavelengths. When a plurality of such amplifiers are deployed on an optical path, the spectral distribution of a transmitted optical signal would be seriously distorted when it arrives at the receiving end. Further, increased crosstalk noise is another problem that is caused by the non-linearity of optical amplifiers. It is difficult to receive the information without solving those problems.
The above-described difficulties with conventional optical amplifiers come from their wavelength-dependent gain characteristics. This is called “gain tilt” in optical communications terminology, which is one of the negative factors that limit the maximum transmission distance of a WDM system. In order to reduce the wavelength dependence of amplifier gains, an optical channel equalizer is inserted in the WDM transmission line, which splits a given WDM signal into individual wavelength components (i.e., into individual channels), gives an appropriate attenuation to each channel, and recombines them into a single optical beam for transmission. To this end, conventional systems employ a plurality of optical attenuators. Such systems, however, need as many attenuator modules as the number of WDM channels, which increases the size and complexity of network equipment.
As a solution for the above problem, one of the inventors of the present invention proposed a variable optical attenuator in the Unexamined Japanese Patent Publication No. 11-119178 (1999), which is the basis of the U.S. Pat. No. 5,999,305 granted to the same inventor. The proposed attenuator uses magneto-optical effect to yield a desired attenuation profile for multiple-channel optical signals. More specifically, a magneto-optical crystal is combined with a means for exposing it in a magnetic field with an arbitrary distribution. This single optical device can provide arbitrary attenuation to each individual optical channel.
FIG. 17
shows the concept of the conventional variable optical attenuator mentioned above. A given WDM signal runs through an optical fiber
410
until it reaches two dispersion devices (gratings)
420
and
430
, where the light is split into individual wavelength components dispersed in the X-axis direction. The resulting parallel rays of light are incident on a magneto-optical crystal
455
with a reflective coating
456
on its back. The rays are reflected at the reflective coating
456
, and the returning light goes back through the same optical path as described above.
The magneto-optical crystal
455
is disposed between permanent magnets
457
a
(S pole) and
457
b
(N pole), so that magnetic saturation will be reached in that crystal
455
. The magneto-optical crystal
455
is further applied a controlled magnetic field generated from an array of main magnetic cores
454
. Here, we can produce any desired magnetic field distribution by commanding a controller
460
to vary electrical current of each individual main magnetic core
454
. The magneto-optical crystal
455
serves as a Faraday rotator, which changes the polarization angle of each optical signal component under the influence of the magnetic field being applied. The Faraday rotation angle of a particular wavelength component is determined by the magnetic field strength at a corresponding portion of the magneto-optical crystal
455
. A birefringent crystal
440
is placed on the optical path, so that the optical signal will be attenuated in accordance with that Faraday rotation angle. The mechanism of
FIG. 17
gives an arbitrary attenuation level to each different wavelength channel in this way.
While the above-mentioned patent application provide almost no details as to the structure of the magnetic field generator
450
, there are a couple of other literatures that analogously suggest how to construct it. Although they are originally designed, not for optical attenuators, but for use in a magnetic display device, we are now going to present those two prior-art examples. Both of them are magnetic write heads that apply vertical magnetic fields on a magnetic display medium.
Referring to
FIG. 13
, a first example of such a conventional magnetic head unit is shown. According to the disclosure in the Unexamined Japanese Patent Publication No. 8-167112 (1996), the body of this unit comprises a flexible circuit board
216
and a housing plate (holding member)
204
made of non-magnetic material. Processed on the housing plate
204
are a plurality of housing cavities
210
each having a side slit
212
. The housing cavities
210
accommodate a plurality of discrete coil units, each being composed of a magnetic core
208
made of magnetic material and a coil
206
with terminals
214
a
and
214
b
. While
FIG. 13
shows them separately, the coil
206
is actually wound around the magnetic core
208
.
Every housing cavity
210
has an opening at the front end of the housing plate
204
and a side slit
212
on the top surface of the same. The air-core coils
206
are inserted through the front openings, together with the magnetic cores
208
, one for each. The terminals
214
a
and
214
b
of each coil
206
are guided out of the housing cavity
210
through the slit
212
and through-holes
217
on the flexible circuit board
216
. Finally, they are connected electrically (e.g., by soldering) to some conductors on the flexible circuit board
216
, which provides wiring to coil driver circuits (not shown). The coils
206
are energized by individual drive currents that are supplied through the wiring on the flexible circuit board
216
, whereby a desired magnetic field is produced in each corresponding magnetic core
208
.
Another example of a conventional magnetic head unit is shown in the Unexamined Japanese Patent Publication No. 11-219507 (1999).
FIG. 14
depicts the structure of this second example, and
FIG. 15
is an enlarged cross-sectional view of part C of FIG.
14
. The illustrated magnetic write head has a plurality of very thin coil units
320
that are arranged side by side on a single plane. More specifically, it i
Fukushima Nobuhiro
Sasaki Seimi
Tachibana Satoshi
Dang Hung Xuan
Fujitsu Limited
Martinez Joseph
Staas & Halsey , LLP
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